release-26.2: mmaprototype: materialize per-range constraint analysis in snapshot

[tbg]

Backport 1/1 commits from #169747 on behalf of @tbg.

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Add a RangeAnalysis sub-message to RangeSnapshot exposing the most useful pieces of mmaprototype.rangeAnalyzedConstraints:

• voter_balance / non_voter_balance — (current count) - (configured count). Zero means the range has the configured number of voters / non-voters; positive means surplus, negative means missing. Framed this way so a fully-conforming range yields the proto3 zero value and serializes as an empty analysis: {} block.
• unsatisfied_constraint_voter/non_voter_store_ids — replicas that satisfy no SpanConfig.Constraints conjunction (MMA's preferred eviction candidates).
• unsatisfied_voter_constraint_store_ids — voters that satisfy no SpanConfig.VoterConstraints conjunction.
• leaseholder_lease_preference_index — the earliest LeasePreferences entry matched by the current leaseholder.

These values are derivable from the rest of the snapshot (SpanConfig + Replicas + per-store StoreAttributes), but offline recomputation requires reimplementing MMA's constraint-matching logic (~30 LOC) and lease-preference scoring, which every consumer would otherwise duplicate. Materializing them keeps the snapshot self-describing for "is this range conforming?" / "is the lease in the right region?" diagnostics without coupling consumers to MMA's internal matcher. The fields are framed so that the typical fully-conforming range carries no payload at all — every list is empty, both balances are zero, and the leaseholder lease-preference index is zero (which doubles as the "no preferences configured" sentinel) — so the analysis only takes up snapshot space when there is something noteworthy to report.

The analysis is computed directly from rangeState.replicas and rangeState.conf rather than reading the rangeState.constraints cache, which is built lazily by the allocator and may be unpopulated. LEARNER and VOTER_DEMOTING_LEARNER replicas are excluded from both buckets, matching the source-side rangeAnalyzedConstraints semantics.

Epic: none
Release note: None

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Release justification:

[trunk-io]

😎 Merged directly without going through the merge queue, as the queue was empty and the PR was up to date with the target branch - details.

[blathers-crl]

Thanks for opening a backport.

Before merging, please confirm that the change does not break backwards compatibility and otherwise complies with the backport policy. Include a brief release justification in the PR description explaining why the backport is appropriate. All backports must be reviewed by the TL for the owning area. While the stricter LTS policy does not yet apply, please exercise judgment and consider gating non-critical changes behind a disabled-by-default feature flag when appropriate.

[cockroach-teamcity]

This change is